1. The Field of the Invention
The present invention relates to a tellurite glass optical fiber and its fabrication method, the optical fiber having its zero dispersion wavelength controlled in an optical telecommunication window and having high nonlinearity. More specifically, the present invention relates to an optical fiber and its fabrication method, the optical fiber having its zero dispersion wavelength in a 1.2-1.7 μm band, an optical telecommunication window, which is implemented by designing the refractive index, structure and material of the tellurite glass fiber.
2. The Relevant Technology
Recently, studies of increasing capacity of optical communication systems have been made because of the explosive growth in communication demand due to a rapid proliferation of the Internet, and to the demand for cost reduction of optical communication systems. In addition to time division multiplexing transmission systems which have been studied as means for increasing the capacity, wavelength division multiplexing (WDM) transmission systems, which transmit signal lights with different wavelengths by multiplexing them onto a single optical fiber, have been developed and spread at an increasingly fast pace. The WDM transmission systems can multiplex signals with different modulation schemes, and expand the systems using new wavelengths, thereby being able to construct more flexible optical communication systems.
To expand the scale and to improve the functions of a WDM transmission network more flexibly, functional optical devices such as wavelength conversion devices, high-speed optical switches and supercontinuum lightwave sources are essential. In the development of the functional optical devices, nonlinear optical devices have been studied intensively which utilize the nonlinear effect in optical fibers.
The amount of production of the nonlinear effect in an optical fiber is proportional to a nonlinear optical coefficient γ. The nonlinear optical coefficient γ has the following relationship between an effective core cross sectional area Aeff and a nonlinear refractive index n2.γ∝n2/Aeff 
Accordingly, to achieve a large nonlinearity, it is necessary to use an optical material with a large nonlinear refractive index n2 and to make Aeff small. Here, the effective core cross sectional area Aeff is given by the following expression (for example, see non-patent document 1).
      A    eff    =                    (                              ∫                          -              ∞                        ∞                    ⁢                                    ∫                              -                ∞                            ∞                        ⁢                                                                                                  F                    ⁡                                          (                                              x                        ,                        y                                            )                                                                                        2                            ⁢                              ⅆ                x                            ⁢                              ⅆ                y                                                    )            2                      ∫                  -          ∞                ∞            ⁢                        ∫                      -            ∞                    ∞                ⁢                                                                          F                ⁡                                  (                                      x                    ,                    y                                    )                                                                    4                    ⁢                      ⅆ            x                    ⁢                      ⅆ            y                              
Many of the silica glass nonlinear optical fibers currently reported increase the nonlinear refractive index of the silica glass itself by doping germanium or the like to the core to increase the nonlinearity, and decrease the effective core cross sectional area by increasing the relative refractive-index difference by doping fluorine to the cladding. In addition, to produce the nonlinear effect at high efficiency in the optical telecommunication window, the zero dispersion wavelength of the optical fibers must be set at 1.2 μm-1.7 μm to fulfill the phase matching conditions.
As for the silica fiber, however, its zero-material dispersion wavelength is about 1.2 μm, and it is difficult to shift the zero-material dispersion wavelength greatly by a dopant. Thus, a method is used which brings the wavelength dispersion value at the 1.55 μm band close to zero by optimizing the structural parameters of the optical fiber (see non-patent document 2, for example).
On the other hand, optical fibers called photonic crystal fiber (abbreviated to PCF from now on) or holy fiber (abbreviated to HF from now on) are now reported which mainly use silica glass and have many air holes formed in the longitudinal direction inside the silica fiber intentionally (see non-patent document 3, for example).
Employing the fiber structure having such air holes can provide a variety of characteristics that cannot be achieved by optical fibers with a conventional core-cladding structure, and hence applications to optical fibers with high nonlinearity are expected.
However, a silica based PCF or HF having a zero dispersion wavelength of 1.2 μm-1.7 μm and high nonlinearity has not yet been implemented. In addition, although the silica glass is superior in the transparency, since its nonlinearity is not so large, it generally lengthens the interaction length to ensure the interaction length needed for the nonlinear effect. For example, long optical fibers of several hundred meters are used sometimes. Thus, realizing more compact nonlinear optical devices with higher efficiency have been much needed which uses optical materials with higher nonlinearity.
Recently, on the other hand, technology development efforts have been conducted for applying tellurite EDFAs (Erbium-Doped Fiber Amplifiers) to an optical communication field. The tellurite refers to tellurite based glass that is predominantly composed of TeO2. The tellurite EDFA, which consists of a erbium-doped tellurite fiber formed by doping erbium to tellurite based glass, is an amplifier that amplifies light by guiding the light wave through the optical fiber by several tens of meters. Using the tellurite EDFA enables the lumped amplification of the wavelength band from 1.53 μm to 1.61 μm which is twice or more wider than the wavelength band from 1.53 μm to 1.56 μm that can be amplified by a conventional silica based EDFA or fluoride EDFA (see non-patent document 1). Furthermore, using the tellurite EDFA enables the fabrication of amplifiers at a wavelength in the 1.6 μm band (see non-patent document 4). Accordingly, the tellurite EDFAs attract attention to be EDFAs for future ultra large capacity WDM systems.
As shown in FIG. 1, the cross section of an optical fiber 4 for an optical amplifier composed of conventional tellurite glass includes a circular core 1 placed at its center, a cladding 2 covering the core's surroundings concentrically, and a jacket 3 further cloaking the cladding's surroundings concentrically. FIG. 2 shows a refractive-index profile of the optical fiber 4. Assume that the difference between the refractive index of the core 1 and the refractive index of the cladding 2 is Δ1, the difference between the refractive index of the core 1 and the refractive index of the jacket 3 is Δ3, and the difference between the refractive index of the cladding 2 and the refractive index of the jacket 3 is Δ2, then Δ1 is much greater than Δ2, thereby strongly confining the light within the core 1.
In the optical fiber 4, the core 1 is doped with a dopant so that the refractive index of the core 1 is sufficiently greater than the refractive index of the cladding 2. Thus, a light beam travels through the core 1 with carrying out total reflection at the interface between the core 1 and cladding 2. In addition, the dispersion can be controlled to some extent by varying the refractive index of the core 1 and the diameter of core 1. However, the single mode condition is not met when the diameter of the core 1 is increased. This results in a multi-mode optical fiber having a plurality of modes, which deteriorates the transmission characteristics. In contrast, when the diameter of the core 1 is decreased, matching of connection with other devices cannot be made. For these reasons, it is impossible for the conventional tellurite glass optical fiber to establish the control range of the dispersion.
Since the tellurite glass has large third order nonlinearity (see, non-patent document 5), it is expected to apply the tellurite glass to such as pulse compression, optical parametric amplification (OPA), and third harmonic generation (THG). Here, the wavelength at which the material dispersion value of the tellurite glass becomes zero is located at a wavelength band longer than 2 μm.
The wavelength dispersion value of a high NA (Numerical Aperture) fiber used for an optical amplifier at 1.55 μm band is usually of the order of −100 ps/km/nm. Accordingly, the wavelength dispersion value becomes a large value of the order of −1 ps/nm even when a short optical fiber of about 10 m is used.
To apply the optical fiber to a long distance, or to high-speed wavelength division multiplexing transmission, it is necessary to bring the wavelength dispersion value of the optical fiber as close to zero as possible. In contrast, the zero dispersion wavelength of the tellurite glass optical fiber is at the wavelength band beyond 2 μm as mentioned above. Accordingly, the tellurite glass optical fiber cannot make the wavelength dispersion value zero at the 1.55 μm band even if the optimum technique based on the well-known structural dispersion is used which is applied to silica fibers.
Therefore it is difficult to implement the foregoing application in the present optical fiber telecommunication window by utilizing the high nonlinearity of the tellurite glass.
The above-mentioned PCF (or HF) is divided into two types according to the waveguide principle. One of them is a photonic bandgap PCF that confines a light beam by a photonic bandgap. The PCF has a structure including a periodic air hole disposition and a uniform air hole size. The other of them is a refractive index waveguide PCF that confines a light beam by the total reflection achieved by effective refractive index of a medium having air holes. The refractive index waveguide PCF has a structure that does not necessarily have the periodic air hole disposition or the uniform air hole size.
Such PCF or HF can make the refractive index difference greater than the conventional optical fiber by an order of magnitude, thereby being able to achieve large structural dispersion. Because of the structural dispersion, the silica based PCF or HF has its zero dispersion wavelength shifted to a shorter wavelength side. M. J. Gander et al. empirically measured dispersion characteristics of a silica glass optical fiber consisting of a core without air holes and a cladding having air holes disposed hexagonally, and disclosed the results in the non-patent document 6. According to the document, the dispersion value at the 813 nm band was about −77 ps/km/nm. In addition, Birks et al. calculate the dispersion of a PCF, an optical fiber composed of a single material, and advocate the effect of the dispersion compensation of the PCF in the non-patent document 7. Thus, the PCF structure or HF structure is expected to be one of the dispersion compensation methods of tellurite glass optical fibers.
N. G. R. Broderick et al. disclosed fibers with a PCF structure or HF structure using multi-component glass in the patent document 1. The document refers to the tellurite glass as an example of the multi-component glass, and shows that it is a composition of components selected from Na2O, Li2O, Al2O3, CaO, Ga2O3, GeO2, As2O3, SrO2, Y2O3, Sb2O5, In2O3, ZnO, BaO, La2O3, TeO2 and TiO2. However, the patent document 1 does not refer to the thermal stability or nonlinear characteristics of the glass or to the dispersion of the tellurite fiber.
E. S. Hu et al. designed a PCF structure or HF structure using the tellurite glass, and disclosed fibers that shift the zero dispersion wavelength to 1.55 μm in the non-patent document 8. The document discloses that three different PCF structures or HF structures were formed using tellurite glass with a zero-material dispersion wavelength of 1.7 μm, and that each structure was able to shift the zero dispersion wavelength to 1.55 μm. As for the fibers disclosed in the non-patent document 8, however, since the tellurite glass used have low nonlinear susceptibility, and the zero-material dispersion wavelength is 1.7 μm, the optical confinement within the core region is insufficient, and hence it is impossible to obtain sufficiently large nonlinearity (the nonlinear coefficient γ reported was 260 W−1 km−1 at the maximum).
The tellurite glass has large third order nonlinearity. Accordingly, systems utilizing optical fibers composed of the tellurite glass having the high nonlinearity have been studied. For example, as shown in FIG. 3, it has been proposed to utilize an optical fiber 8, which has a core 5 and a cladding 6 composed of tellurite glass, for optical amplification such as a Raman amplifier (see non-patent document 9, for example).
In addition, the limit at which the gain is achieved on the longer wavelength side of the tellurite EDFA is increased by 7-9 nm compared with a silica based EDFA or fluoride EDFA. This enables an amplifier at a 1.6 μm band wavelength which cannot be utilized conventionally (see non-patent document 4, for example). Consequently, the tellurite EDFAs attract attention as EDFAs in the future super large capacity WDM transmission systems.
Fibers using the tellurite glass have been applied to Er3+-doped fiber amplifiers or Raman amplifiers, and implement wideband amplifiers (see non-patent document 1 and non-patent document 8). The tellurite glass has nonlinear effect 10 or more times greater than that of the silica glass, and at the same time implements low loss fibers with a loss of 20 dB/km in the application to the Raman amplifier. Thus, the tellurite glass has wideband optical amplification characteristics and high transparency. In addition, the tellurite glass has large optical nonlinear susceptibility χ3 (see non-patent document 5, for example). Accordingly, nonlinear devices are expected which are more compact and have higher efficiency than ever.
However, it is difficult for the tellurite glass optical fibers to satisfy the phase matching condition between the pumping light and the 1.55 μm band signal light, which is the optical telecommunication window, because the wavelength at which the material dispersion becomes zero is located in a wavelength band longer than 2 μm, thereby making it difficult to utilize the nonlinearity positively. For example, the tellurite glass optical fibers used for optical amplifiers have a wavelength dispersion value of about −100 ps/km/nm at the wavelength 1.55 μm.
A dispersion-shifted optical fiber or dispersion compensation optical fiber controls the dispersion by increasing the relative refractive-index difference between the core and the cladding by applying the structure of the conventional optical fiber. Applying the method to the tellurite glass optical fiber, however, causes the zero dispersion wavelength to be further shifted to a longer wavelength side. Accordingly, it is very difficult for the tellurite glass optical fiber to implement the zero dispersion at the 1.55 μm band which is the optical telecommunication window. As a result, a communication system cannot be implemented which utilizes the optical fiber composed of the tellurite glass with high nonlinearity.
As for a fabrication method, an extrusion process is reported as a fabrication method of a photonic crystal fiber or holy fiber composed of oxide glass other than the silica-based glass (see non-patent document 10, and non-patent document 11). The extrusion process fabricates a preform having air holes by heating fabricated bulk glass to a high temperature at which it has deformable viscosity, and by pressing it into a mold, followed by extruding it. It is difficult for the extrusion process to fabricate a low loss fiber because the glass is kept at a high temperature for a long time and undergoes deformation, and hence crystal nuclei are apt to grow in the glass. Accordingly, loss values of fibers disclosed in the non-patent documents 10 and 11 each exceed 1000 dB/km, and no fibers have been implemented which have a loss usable as practical devices.    Patent document 1: EP1313676, USP2003/0161599 “Holy optical fiber of non-silica based glass” Southampton University.    Patent document 2: Japanese Patent Application Laid-open No. 2003-149464.    Patent document 3: Japanese Patent Application Laid-open No. 2000-356719.    Non-patent document 1: A. Mori, Y. Ohishi, M. Yamada, H. Ono, Y. Nishida, K. Oikawa, and S. Sudo, “1.5 μm broadband amplifier by tellurite-based DFAs”, in OFC'97, 1997, Paper PD1.    Non-patent document 2: Shojiro Kawakami, Kazuo Shiraishi, and Masaharu Oohashi, “Optical fiber and fiber mold devices”, Baifuukan, Inc. p. 97.    Non-patent document 3: A. Bjarklev, et al., “Photo Crystal Fibers The State of The Art”, Holy fibers Symposium vol. 1.1, ECOC2002.    Non-patent document 4: A. Mori, Y. Ohishi, M. Yamada, H. Ono and S. Sudo, “Broadband amplification characteristics of tellurite-based EDFAs”, in ECOC '97, vol. 3, 1997, Paper We2C.4, pp. 135-138.    Non-patent document 5: S. Kim, T. Yoko and S. Sakka, “Linear and Nonlinear Optical Properties of TeO2 Glass”, J. Am. Ceram. Soc., Vol. 76, No. 10, pp. 2486-2490, 1993.    Non-patent document 6: M. J. Gander, R. McBride, J. D. C. Jones, D. Mogilevtsev, T. A. Birks, J. C. Knigth, and P. St. J. Russell, “Experimental measurement of group velocity dispersion in photonic crystal fibre”, Electron. Lett., January 1999, vol. 35, no. 1, pp. 63-64.    Non-patent document 7: T. A. Birks, D. Mogilevtsev, J. C. Knight, P. St. J. Russell, “Endlessly single-mode photonic crystal fiber-Dispersion compensation using single-material fibers” Opt. Lett. 22, 1997, pp. 961-963.    Non-patent document 8: ECOC2002 nonlinearity-Parametric Amplifiers 3.2.3 “Design of Highly-Nonlinear tellurite fibers with Zero Dispersion Near 1550 nm” Stanford University.    Non-patent document 9: “Journal of Lightwave Technology”, 2003, Vol. 21, No. 5, pp. 1300-1306.    Non-patent document 10: P. Petropoulos, et al., “Soliton-self-frequency-shift effects and pulse compression in an anomalously dispersive high nonlinearity lead silicate holy fiber”, PD3-1, OFC2003.    Non-patent document 11: V. V. Ravi Kanth Kunth, et al., “Tellurite glass photonic crystal fiber” PD3 ECOC2003.    Non-patent document 12: Gorachand Ghosh, “Sellmeier Coefficients and Chromatic Dispersions Dispersiond for Some Tellurite Glasses”, J. Am. Soc., 78(10) 2828-2830, 1995.    Non-patent document 13: “Photonics Technology Letters”, 1999, Vol. 11, No. 6, pp. 674-676.    Non-patent document 14: A. Mori, et al., “Ultra-wideband tellurite-Based Raman fibre amplifier”, Electronics Letter vol. 37, No. 24, pp. 1442-1443, 2001.    Non-patent document 15: Govind P. Agrawal, “Nonlinear Fiber Optics”, 2nd edition, Academic Press, pp. 42-43.